1. Field of the Invention
The present invention relates to a photoreceptor array applicable to an optical encoder and method of manufacturing the same.
2. Description of the Related Art
A small optical encoder may include a sensor head that employs an array of photoreceptors also serving as index gratings. This photoreceptor array can provide four-phase displacement signals if four photoreceptors are arrayed in a set with a pitch of (2nxe2x88x921)xcex/4 where xcex is a pitch in scale gratings and n is a positive integer.
Such the photoreceptor array can be produced from photodiodes that are fabricated in a single crystal silicon substrate. In this case, however, the smaller the pitch in the array of photoreceptors the larger the crosstalk between photoreceptors adjoining with each other via the substrate. In order to produce a photoreceptor array without such the crosstalk, it is desirable to form photodiodes using amorphous silicon on an insulating substrate such that each photodiode is isolated from others. An array of pin photodiodes can be obtained when amorphous silicon of p-, i- and n-types are layered on the insulating substrate and subsequently etched.
If an interval between PDs is as fine as 4 xcexcm or below, the method of manufacturing a PD (photodiode) array using the etching of amorphous silicon layers makes an aspect ratio larger, remains etching residues easily and causes short faults between adjacent PDs. A dry etching such as a plasma etching may be employed for fine etching of amorphous silicon. This etching process often imparts damages on PDs and diffuses impurities from sidewalls into PDs. For these reasons, the conventional method can not achieve an excellent PD property and high yield.
The present invention has an object to provide a photoreceptor array with an excellent device property and no short fault between adjacent photoreceptors and to provide a method of manufacturing such the photoreceptor array with a high yield.
The present invention is provided with a photoreceptor array, which comprises a substrate; an insulating layer formed on the substrate, the insulating layer having a plurality of trenches formed therein for burying devices; a plurality of photoreceptors formed from semiconductor layers buried in each of the trenches in the insulating layer; and an output signal line formed on the plurality of photoreceptors via an interlayer insulator.
The present invention is also provided with a method of manufacturing a photoreceptor array, which comprises the steps of providing a substrate; forming an insulating layer on the substrate; forming a plurality of trenches in the insulating layer; forming a plurality of photoreceptors from semiconductor layers buried in each of the trenches in the insulating layer; and forming an output signal line on the plurality of photoreceptors via an interlayer insulator.
According to the present invention, the photoreceptor array is formed from the semiconductor layers buried in the trenches in the insulating layer on the substrate. Therefore, adjacent photoreceptors in the array can be isolated from each other reliably with no problem to cause a short fault therebetween. Accordingly, a fine pitch array of photoreceptors can be obtained with an excellent device property and high yield.
In an embodiment, the substrate may be composed of a transparent substrate. In addition, the photoreceptor array may further comprise a transparent electrode that is formed between the transparent substrate and the insulating layer and is operative as a lower electrode common to the plurality of photoreceptors. In this photoreceptor array, the back surface of the substrate is employed to receive a light. If the photoreceptors have upper electrodes, and each of which is composed of a transparent electrode, the upper electrodes may be employed to receive a light from above. In this photoreceptor array, the substrate is not required transparent and the lower electrode may be a metallic electrode.
Preferably, the plurality of photoreceptors may have upper electrodes, each of which may be self-aligned with and buried in the trench. This can prevent short faults between photoreceptors in contrast to the case where the upper electrodes are etched for pattern formation.
In an embodiment, the photoreceptor may be a pin or pn photodiode (PD). In this case, p-type layers in the plurality of PDs may be formed as a single and common p-type layer continuously on the common lower electrode. An i-type layer and an n-type layer are buried and formed in each trench.
The present invention is further provided with an optical encoder, which comprises a scale having optical gratings formed thereon along a measurement axis; and a sensor head including a photoreceptor array for detecting displacements of the scale to provide a plurality of displacement signals with different phases. The photoreceptor array includes a substrate; a lower electrode formed on the substrate; an insulating layer formed on the substrate, the insulating layer having a plurality of trenches formed therein for burying devices; a plurality of photoreceptors formed from semiconductor layers buried in each of the trenches in the insulating layer, the plurality of photoreceptors each having an upper electrode formed thereon; and an output signal line formed on the plurality of photoreceptors via an interlayer insulator.
According to the present invention, a high performance optical encoder with a fine scale pitch can be obtained.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof.